Additional note: it seems a casualty of Mozilla doing this has been the 12-character authentication (J-PAKE) service for linking devices. I'll be looking at putting a key exchange server in production soon to remedy this.
In the meantime, you will have to enter your Pale Moon Sync user name
and recovery key
on the new device.
EDIT: Confirmed by a Mozilla employee that the key exchange server was shut down as well as the storage servers @Mozilla, since it was apparently only used by Sync 1.1 on their infra (even though it could have been used by other services). It was not obvious from their communication that this would happen, or I'd have put more effort into getting this up before their Sync shutdown.
Note that updating this will not be possible except with a new release of the browser, especially for Android. We may yet need 25.7.3, after all
"There will be times when the position you advocate, no matter how well framed and supported, will not be accepted by the public simply because you are who you are." -- Merrill Rose